Is it obligatory for the father who suffers from urinary incontinence to wash off impurities and change his clothes before every prayer, considering the difficulty of that?

If your father cannot control his urine, or cannot change his soiled clothes, then he prays according to his ability. He is excused from the impurity that has afflicted him and performs tayammum for each prayer. However, if he is able to wash away the impurity or change the soiled garment for a pure one at the time of prayer, then he is obligated to do so, as Allah Almighty says: (So fear Allah as much as you are able).